Almost every pasta recipe begins with the same instruction: bring a large pot of water to a boil and add salt. It is such a familiar step that many people do it without asking why. The answer is simpler than some kitchen myths suggest, and it has more to do with flavor than with temperature.
The real reason: seasoning from the inside
Dry pasta is made mostly from flour and water, and on its own it tastes quite plain. As it cooks, the pasta absorbs water and swells. If that water is salted, a little of the salt travels into the pasta along with it. The result is noodles that are seasoned all the way through, instead of plain noodles with salty sauce sitting on the surface.
This is hard to fix later. Sprinkling salt on cooked pasta mostly seasons the outside, and adding more salt to the sauce does not quite reach the center of each piece. That is why many cooks consider salting the water one of the easiest ways to make a simple dish taste more complete.
What about the boiling point?
You may have heard that salt makes water boil at a higher temperature so that pasta cooks faster. Dissolved salt does raise the boiling point of water, but at the amounts used in cooking the change is only a small fraction of a degree. You would not notice the difference in cooking time. The salt is there for taste.
Why the water sometimes fizzes
If you pour salt into water that is already boiling, it often foams up for a moment. The salt grains give dissolved gas and steam bubbles new places to form, so a burst of bubbles rises all at once. It is harmless, but it is a good reason to add salt carefully and keep your hand away from the steam.
How much salt?
There is no single correct amount. Some cooks like their pasta water noticeably salty, while others prefer a lighter touch, especially if the sauce will be salty already, as with cheese, olives, or cured meats. A practical approach is to start with a modest amount, taste a piece of cooked pasta, and adjust next time. Your own taste is the best guide.
A bonus: save a cup
Before you drain the pasta, scoop out a cup of the cooking water. It contains starch released by the pasta, and a splash of it can help a sauce become smoother and cling better to the noodles. Because the water is already seasoned, it adds flavor rather than diluting it.
One small pinch at the start, in other words, quietly shapes the whole dish.
